android - FFT implementation produces a glitch -

i'm getting strange glitch in fft graph white noise:

i've checked reference program , while noise file seems fine. bug in implementation?

void four1(float data[], int nn, int isign) {     int n, mmax, m, j, istep, i;     float wtemp, wr, wpr, wpi, wi, theta;     float tempr, tempi;      n = nn << 1;     j = 1;     (int = 1; < n; += 2) {         if (j > i) {             tempr = data[j];             data[j] = data[i];             data[i] = tempr;             tempr = data[j + 1];             data[j + 1] = data[i + 1];             data[i + 1] = tempr;         }         m = n >> 1;         while (m >= 2 && j > m) {             j -= m;             m >>= 1;         }         j += m;     }     mmax = 2;     while (n > mmax) {         istep = 2 * mmax;         theta = twopi / (isign * mmax);         wtemp = sin(0.5 * theta);         wpr = -2.0 * wtemp * wtemp;         wpi = sin(theta);         wr = 1.0;         wi = 0.0;         (m = 1; m < mmax; m += 2) {             (i = m; <= n; += istep) {                 j = + mmax;                 tempr = wr * data[j] - wi * data[j + 1];                 tempi = wr * data[j + 1] + wi * data[j];                 data[j] = data[i] - tempr;                 data[j + 1] = data[i + 1] - tempi;                 data[i] += tempr;                 data[i + 1] += tempi;             }             wr = (wtemp = wr) * wpr - wi * wpi + wr;             wi = wi * wpr + wtemp * wpi + wi;         }         mmax = istep;     } } 

apart few minor changes, code appears taken out of 2nd edition of numerical recipes in c. documentation function (taken book) states:

replaces data[1..2*nn] discrete fourier transform, if isign input 1; or replaces data[1..2*nn] nn times inverse discrete fourier transform, if isign input −1. data complex array of length nn or, equivalently, real array of length 2*nn. nn must integer power of 2 (this not checked for!).

this implementation yields correct results, given input array 1-based indexing. can choose use same indexing convention allocating c array of size 2*nn+1 , filling array starting @ index 1. alternatively pass array of size 2*nn has been fill starting @ index 0, calling four1(data-1, nn, isign) (notice -1 offset on data array).